1. Field of the Invention
The present invention relates to a waveguide-type optical multiplexer/demultiplexer, and more specifically to a waveguide-type optical multiplexer/demultiplexer provided with low crosstalk characteristics and reduced wavelength dispersion.
2. Prior Art
An interleave system which is one of wavelength multiplexing communications in next generation requires an optical multiplexer/demultiplexer provided with a function that a signal with certain channel wavelength spacing is demultiplexer to two signals with twice of said channel wavelength spacing, or contrary to this, two signals are multiplexed to one signal.
FIG. 1(a) and FIG. 1(b) show an example such optical multiplexer/demultiplexer. FIG. 1(a) shows an arrangement of first to third optical multiplexer/demultiplexer circuits 21a, 21b, 21c each of which is a constituent unit of this sort of optical multiplexer/demultiplexer and has demultiplexer wavelength λN on a quartz substrate 20, and FIG. 1(b) shows connecting structure of first to third optical multiplexer/demultiplexer circuits 21a, 21b, 21c. 
FIG. 2 shows detailed constitution of one of these optical multiplexer/demultiplexer circuits 21a, 21b, 21c connected in multistage. Numeral 30 denotes a first waveguide provided with phase difference giving portions 14, 17, 18 which arise interference at predetermined portion, and numeral 40 denotes a second waveguide provided with phase difference giving portions 15, 16, 19 which arise interference at the same portion as the first waveguide 30.
Numerals 10 to 13 denote optical coupling portions which couple the first waveguide 30 and the second waveguide 40 each other at predetermined portions. By coupling the first waveguide 30 and the second waveguide 40 as explained above, Mach-Zehnder interference circuit is constituted. Coupling efficiency at optical coupling portions 10, 11 is about 50%, and at optical coupling portions 12, 13 is about 3.5%. In FIG. 2, numerals 6 to 9 denote respectively first to fourth input-output ports each of which is formed at ends of first and second waveguides 30, 40.
According to above constitution, first to fourth input-output ports A, B, C, D of first to third optical multiplexer/demultiplexer circuits 21a, 21b, 21c in FIG. 1(a) correspond respectively to first to fourth input-output ports 6, 7, 8, 9 in FIG. 2. Further, input port 22 in FIG. 1(a) and FIG. 1(b) corresponds to input port 6 in FIG. 2, and output ports 23, 24 in FIG. 1(a) and FIG. 1(b) correspond to output port 7 in FIG. 2. Furthermore, ΔL1, ΔL2 and ΔL3 in FIG. 1(b) denote optical path difference which are given respectively between phase difference giving portions 15 and 14, between 16 and 17, and between 19 and 18 by formation of phase difference giving portions 14, 15, 16, 17, 18 and 19 in FIG. 2.
With respect to above explained first to third optical multiplexer/demultiplexer circuits 21a, 21b, 21c shown in FIG. 1(a), when wavelength division multiplex signals of wavelength λ1, λ2, λ3, λ4, λ5, λ6 . . . are input through the first input-output port A, multiplex signals of odd channel wavelength λ1, λ3, λ5 . . . are output from the third input-output port C, and multiplex signals of even channel wavelength λ2, λ4, λ6 . . . are output from the fourth input-output port D.
On the other hand, when wavelength division multiplex signals of wave length λ1, λ2, λ3, λ4, λ5, λ6 . . . are input through the second input-output port B, multiplex signals of odd channel wave length λ1, λ3, λ5 . . . are output from the fourth input-output port D, and multiplex signals of even channel wavelength λ2, λ4, λ6 . . . are output from the third input-output port C.
In the optical multiplexer/demultiplexer shown in FIG. 1(a), the first input-output port A of the optical multiplexer/demultiplexer circuit 21a is selected as the whole input port, further, the third input-output port C of the first optical multiplexer/demultiplexer circuit 21a is connected to the fourth input-output port D of the second optical multiplexer/demultiplexer circuit 21b, and the fourth input-output port D of the first optical multiplexer/demultiplexer circuit 21a is connected to the third input-output port C of the third optical multiplexer/demultiplexer circuit 21c. 
Further referring to FIG. 1(b), defining that wavelength within using wavelength range is λs and equivalent refractive index of waveguide at wavelength λs is Neff, and assuming that ΔL1 is determined by multiplexing/demultiplexing wavelength, relations ΔL2=2×ΔL1 and ΔL3=4×ΔL1−λs/(Neff×2) are given.
Concretely, in order to obtain operating wavelength range from 1520 nm to 1620 nm, ΔL1 is set about 1 mm, ΔL2 is set about 2 mm, ΔL3 is set about 4 mm, and spacing of wavelength λ1, λ2, λ3, λ4, λ5, λ6 . . . is set about 0.8 nm (100 GHz in frequency).
FIG. 3 shows wavelength dispersion characteristics and permeable index characteristics of channel path band at input-output ports 22, 23 of optical multiplexer/demultiplexer circuits 21a, 21b in above described prior art waveguide-type optical multiplexer/demultiplexer. According to the figure, permeable index characteristics between optical multiplexer/demultiplexer circuits 21a and 21b is identical, however, wavelength dispersion characteristics between optical multiplexer/demultiplexer circuits 21a and 21b is identical in absolute value but reverse in phase. Further, because characteristics of optical multiplexer/demultiplexer circuits 21a and 21b are added as the whole optical multiplexer/demultiplexer, wavelength dispersion in this example is characterized to zero in principle,
Accordingly, the above described prior art waveguide-type optical multiplexer/demultiplexer (hereinafter referred to as “prior art 1”) is appraised as a optical multiplexer/demultiplexer provided with superior characteristics, on the other hand, considering application to actively developing transmission rate 40 Gbps class system, crosstalk characteristic is not necessarily sufficient.
Referring to FIG. 4, crosstalk from channel of center wavelength λn+1 to channel of center wavelength λn is considered. In a system that high speed modulation such as transmission rate 40 Gbps is carried out, because influence of the modulating side wave band affects to adjacent channels, low crosstalk characteristic is required even for wavelength apart from the center wavelength. However, in the optical multiplexer/demultiplexer of prior art 1, crosstalk of 0.35 nm apart from center wavelength is 17 dB. This crosstalk level is so high that may cause problem in practical use.
An optical multiplexer/demultiplexer (hereinafter referred to as “prior art 2”) as shown in FIG. 5(a) and FIG. 5(b) is proposed to improve the high crosstalk characteristic. This optical multiplexer/demultiplexer is almost same construction as that shown in FIG. 1(a) and FIG. 1(b), however, in order to improve crosstalk characteristic, demultiplexer wavelength λn of the first optical multiplexer/demultiplexer circuit 26a is shifted Δλ=0.15 nm to short wave direction in respect of the first optical multiplexer/demultiplexer circuit 21a of prior art 1, and demultiplexer wavelength λn of the second and third optical multiplexer/demultiplexer circuits 26b, 26c are shifted Δλ=0.15 nm to long wave direction in respect of the second and third optical multiplexer/demultiplexer circuits 21b, 21c of prior art 1.
Concretely, as shown in FIG. 5(b), optical path difference ΔL1, ΔL2 and ΔL3 of the optical multiplexer/demultiplexer circuits 26(a), 26b, 26c are equivalently shortened or lengthened δ1, δ2 and δ3 respectively. For example, δ1 is set about 0.1 μm, δ2 is set about 0.2 μm and δ3 is set about 0.4 μm. Wherein equivalently means either to change waveguide length or to change optical path length by changing refractive index of waveguide without changing waveguide length. In FIG. 5(a) and FIG. 5(b), numeral 25 denotes a quartz substrate, numeral 27 denotes a input port and numerals 28, 29 denote output ports.
In the waveguide-type optical multiplexer/demultiplexer of prior art 2 as described above, if crosstalk characteristic level is estimated based on FIG. 4, crosstalk of 0.35 nm apart from center wavelength is 27 dB. This crosstalk level is confirmed to be sufficient characteristic in practical use.
However, in accordance with the waveguide-type optical multiplexer/demultiplexer of prior art 2, in spite of obtaining excellent crosstalk characteristics, defect of increasing wavelength dispersion is appeared.
FIG. 6 shows wavelength dispersion characteristics and permeable index characteristics of channel path band at output port 28 of prior art 2. As shown in the figure, wavelength dispersion of optical multiplexer/demultiplexer circuits 26a and 26b is increased near the center of channel, further, characteristics of optical multiplexer/demultiplexer circuit 26a and 26b are added as a whole optical multiplexer/demultiplexer, accordingly wavelength dispersion at the center of channel shows nearly 30 ps/nm.
This dispersion value is obviously high level and a serious problem, especially, in high speed transmission system such as 40 Gbps transmission rate.